Vitamin D3 is a vitamin that is an essential component of biological regulating systems in humans. Sun exposure is the predominant source of vitamin D3. Previous research has shown that vitamin D3 deficiency is common worldwide. It is especially common in northern countries with long winters due to inadequate sun exposure during winter. In the US, an estimated 36% to 57% of healthy middle-aged to elderly adults have vitamin D3 deficiency. Current research indicates that obesity is associated with a low vitamin D3 level.